Electrophoretic imaging apparatus including means to coat and electrify the imaging electrode

ABSTRACT

IT HAS BEEN DETERMINED THAT BY APPLYING DC VOLTAGE TO A COATING MEANS IN THE ELECTROPHORETIC IMAGING SYSTEM THAT IT IS POSSIBLE TO ESTABLISH THE NECESSARY FIELD FOR IMAGING AS WELL AS ELIMINATING THE BUILDUP OF UNWANTED RESIDUAL CHARGE ON THE SURFACE OF THE BLOCKING ELECTRODE.   IN ADDITION, THE APPLICATION OF THE POTENTIAL IN THIS MANNER HAS BEEN FOUND TO DECREASE MOTTLING OF THE IMAGING SUSPENSION ON THE SURFACE OF THE BLOCKING ELECTRODE.

Oct. 3, 1972 L. M. CARREIRA 3,696,020 ELECTROPHORETIC IMAGING APPARATUSINCLUDING MEANS T0 COAT AND ELECTRIFY THE IMAGING ELECTRODE OriginalFiled Jan. 2, 1969 I9 My ,5 z:- 1: a g K 8/ FIG. 2 I

INVENTOR. LEONARD M. CARREIRA A Md, M

ATTORNEY United States Patent O ABSTRACT OF THE DISCLOSURE It has beendetermined that by applying DC voltage to a coating means in theelectrophoretic imaging system that it is possible to establish thenecessary field for imaging as well as eliminating the buildup ofunwanted residual charge on the surface of the blocking electrode. Inaddition, the application of the potential in this manner has been foundto decrease mottling of the imaging suspension on the surface of theblocking electrode.

This is a division of application Ser. No. 789,088, filed in the UnitedStates, Jan. 2, 1969 now Pat. No. 3,586,615.

BACKGROUND OF THE INVENTION This invention relates to an imaging systemand more specifically to an improved photoelectrophoretic imagingsystem.

In photoelectrophoretic imaging colored photosensitive particles aresuspended in an insulating carrier liquid. This suspension is placedbetween at least two electrodes one of which is generally conductive andreferred to as the injecting electrode and the other which is generallyinsulating and called the blocking electrode. The suspension issubjected to a potential difference and exposed to an image to bereproduced. Ordinarily, in carrying out the process, the imagingsuspension is placed on a transparent electrically conductive plate inthe form of a thin lm and exposure is made through the transparent platewhile a second generally cylindrically shaped biased electrode is rolledacross the suspension. The particles are believed to bear an initialcharge when suspended in the liquid carrier which causes them to beattracted to the transparent base electrode and upon exposure, to changepolarity by exchanging charge with the base electrode so that theexposed particles migrate to the second or roller electrode therebyforming images on both of the electrodes by particle subtraction eachimage being complementary to the other. The process may be used toproduce both polychromatic and monochromatic images. In the latterinstance a single color photoresponsive particle may be used in thesuspension or a number of differently colored photoresponsive particlesmay be used all of which respond to the same wavelength of lightexposure. An extensive and detailed description of thephotoelectrophoretic imaging techniques as described may be found in US.Pat. Nos. 3,383,993, 3,384,488, 3,384,565 and 3,384,566.

In the case of the polychromatic imaging process the imaging suspensionwill contain a plurality of at least two differently colored finelydivided particles in a carrier liquid each of said particles comprisingan electrically photosensitive pigment whose principal light absorptionband substantially coincides with its principal photosensitive response.Thus, the pigment represents both the primary electricallyphotosensitive ingredient and the primary colorant for the specificparticle in suspension. The particles utilized in the polychromaticsystem should 3,696,020 Patented Oct. 3, 1972 preferably have intensepure colors and be highly photosensitive. It is preferred that theparticles migrate with minimum exposure to activating electromagneticradiation and that particles of each color migrate to an equal extentupon exposure to light of the complementary color. When the suspensionis exposed to a multicolored image, particles will migrate to oneelectrode in proportion to the intensity of the light which they absorb.This migration should take place with a minimum of electricalinteraction between the particles of different colors. Thus, it ispreferred and desired that particles selectively remain on one of theelectrodes in image configuration with unwanted particles migrating tothe other electrode in the system. For example, when a mixturecomprising cyan, magenta and yellow particles is exposed to an image byyellow light, the cyan and magenta particles should migrate leavingbehind an image made up of yellow particles. Similarly, when exposed toa multicolored image, different colored particles absorb light of theircomplementary color in appropriate image areas and migrate therebyleaving a full colored image corresponding to the. original.

This system, using a conductive injecting electrode, a substantiallyinsulating blocking electrode and photosensitive particles dispersed inan insulating carrier liquid between the electrodes, has been found tobe capable of producing excellent images. Most of the polymericmaterials utilized as the blocking layer on the respective electrodevary in resistivity from 10 to 10 ohms cm. or greater. Those materialsin the lower resistivity range, i.e. 10 to 10 ohms cm., such as barytapaper, although capable of producing excellent image are humiditysensitive (in that their resistivities vary depending upon the relativehumidity they are subjected to). As a result, these materials arecleaned between imaging steps. Blocking layers of materials havinghigher resistivities with less humidity sensitivity are thereforegenerally preferred. However, with those materials of higherresistivities of from about 10 ohms cm. or greater, e.g. Myler andTelfion, the image quality progressively decreases particularly Whereseveral images are made using the same electrode in rapid succession.Therefore, while a great many substantially insulating materials havebeen found operative when used as the blocking layer of the blockingelectrode, due to the apparent charge retention on the specific materialduring the image formation step, the quality of the images formed onthese highly insulating materials when recycled is substantiallyreduced. Thus, the overall image quality is reduced as a result of theaccumulation of the undesirable charges on the blocking electrode. Forexample, conventionally used blocking electrode polymers such as Mylarand Tedlar, having resistivities of about 10 and 10 ohms cm.respectively, illustrate the charge retention problem.

SUMMARY OF THE INVENTION It is therefore an object of this invention toprovide an electrophoretic imaging system which will overcome theabove-noted disadvantages.

It is another object of this invention to provide a novelphotoelectrophoretic imaging system.

It is a further object of this invention to provide aphotoelectrophoretic imaging system capable of producing a plurality ofimages in rapid succession of uniform high quality.

Yet, still a further object of this invention to provide anelectrophoretic imaging system capable of utilizing a wide range ofblocking electrode materials.

The foregoing objects and others are accomplished in accordance with thepresent invention generally speaking by providing an imaging apparatusconsisting of an electrode system utilized in conjunction with animaging suspension comprising a plurality of light absorbingphotoelectrophoretic imaging particles in an insulating carrier liquid.The imaging apparatus utilizes at least one electrode in rollerconfiguration, generally the imaging or blocking electrode, and at leastone additional electrode, the injecting electrode. One of theseelectrodes is at least partially transparent to activatingelectromagnetic radiation. Rotatably mounted in close proximity to theroller electrode in the system is a coating means which continuouslyapplies the imaging suspension to the surface of the roller electrode.The imaging suspension is exposed selectively to an electromagneticradiation source through the transparent electrode in the system whilesimultaneously developing an electrical field across the imagingsuspension by applying a DC voltage to the coating means. The coatingmeans in the form of a rod is in surface contact with the blocking layerof the blocking roller electrode. The coating rod coats the surface ofthe roller electrode with a uniform film of the imaging suspension whileat the same time distributing charge to the surface of the blockingelectrode. Imaging is then completed at the surface of the injectingelectrode as a result of the particle migration within the systemwhereby complementary color images are formed on the surfaces of each ofthe electrodes. The roller or imaging electrode is then recoated by theabove mentioned coating device so as to replenish the imaging suspensionwhile simultaneously eliminating or neutralizing buildup of unwantedcharge on the surface of the insulating blocking electrode. In effect,the charging of the coating rod both contact charges the blockingelectrode so as to establish the necessary field between the blockingand injecting electrode during imaging and, in addition, as the rollerrecycles, eliminates the buildup of residual charge on the surface ofthe blocking electrode. In an alternate embodiment of the presentinvention charge buildup on the blocking electrode may be eliminated ina sequential manner in a system where the field is established betweenthe blocking and injecting electrodes by applying the field chargedirectly to the blocking electrode, by first cutting off the potentialto the core of the blocking electrode then applying charge to thecoating rod to neutralize the residual charge on the surface or blockinglayer of the blocking electrode, then cutting oif the potential to thecoating rod and reestablishing the field between the blocking electrodeand the injecting electrode.

It has been determined in the course of the present invention that byapplying a DC voltage directly to the coating device in the abovedescribed imaging apparatus it is possible to eliminate the buildup ofresidual charge on the blocking electrode which effects the quality ofthe image attained when rapid imaging is desired. In addition toneutralizing the residual charge effect on the surface of the blockingelectrode, it is possible to simultaneously establish'the necessaryfield across the imaging suspension for purposes of imaging. In additionto eliminating charge buildup n the surface of the blocking layer thecharging of the coating rod also provides a means of eliminating mottledor a spotted appearance sometimes obtained during the coating of theimaging suspension on the surface of the blocking electrode.

BRIEF DESCRIPTION OF THE DMWINGS The invention illustrated in theaccompanying drawings wherein:

FIG. 1 represents a side sectional view of a simple exemplary system forcarrying out the steps of the present invention; and

FIG. 2 shows a frontal view of the imaging apparatus of the presentinvention.

DETAILED DESCRIPTION OF DRAWINGS Referring now to FIG. 1 there is seen atransparent electrode generally designated 1 which, in this exemplaryinstance, is made up of a layer of optically transparent glass 2overcoated with a thin transparent layer 3 of tin oxide. The latterconfiguration is commercially available under the trade name NESA glass.This electrode will hereinafter be referred to as the injectingelectrode. In close proximity to the injecting electrode 1 is a rotaryelectrode 5 having a conductive central core 11 which is covered with alayer of blocking electrode insulating material 12. The latter electrodeis referred to as the blocking or imaging electrode. A detaileddescription of the improved results and the types of material which maybe used as the blocking layer may be found in detail in US. Pat.3,383,993. A thin layer 4 of the imaging suspension of the presentinvention, which consists of finely divided photosensitive particlesdispersed in an insulating carrier liquid, is coated on the surface ofthe blocking electrode by coating means generally designated 13 whichmeters the application of the imaging suspension so that it is appliedto a uniform thickness. The coating means comprises coating rods 14 and15 with pressure rollers 16 and 17. The coating and pressure rods aresuspended by support 18. The ink reservoir 19 inherently develops on theupper surface of the blocking electrode 5. During this initial part ofthe description of the invention the term photosensitive may be thoughtof as any particle which once attracted to the injecting electrode willmigrate away from it under the influence of an applied electric fieldwhen it is exposed to actinic electromagnetic radiation. The imagingsuspension may also contain a sensitizer and/or binder for the pigmentparticles the latter being at least partially soluble in the suspendingor carrier liquid. As the blocking or imaging roller 5 is passed acrossthe surface of the injecting electrode 1, switch 7 is closed which asthe result of the presence of potential source 8 establishes an electricfield across the imaging suspension 4. The central core of the blockingelectrode is connected to ground. The pigment suspension is exposed byWay of a projection mechanism made of a light source 21, transparency 22and lens system 23. For purposes of this illustration a positive colortransparency is utilized during the process. As the imaging rollerpasses across the surface of the NESA electrode, ink from the reservoir19 is applied in a uniform manner to the surface of the imagingelectrode. Simultaneously, as a result of the application of a DCpotential to the coating rod 14 the necessary electrical field isestablished across the apparatus so as to allow for imaging at thesurface of the injecting electrode and to eliminate the buildup ofresidual charge on the surface of the insulating blocking electrode.Exposure causes the exposed particles originally attracted to theinjecting electrode 1 to migrate through the liquid carrier and adhereto the surface of the imaging electrode 5 leaving behind an image on thesurface of the injecting electrode which is a duplicate of the originaltransparency 22.

Referring now to FIG. 2 there is seen in a frontal view the imagingapparatus of FIG. 1. The coating means 13 comprising the support 18,coating rod 14 and pressure roller 17 applies the imaging suspension 4to the surface of the blocking electrode 5. The injecting electrode 1comprising the conductives substrate 2 and tin oxide coating 3 issituated beneath the blocking electrode 5. Electrical connection is madebetween the coating rod 14 and the NESA or injecting electrode 1.

It is preferred that the injecting electrode be composed of an opticallytransparent material such as glass overcoated with a conductive materialsuch as tin oxide, copper, copper iodide, gold or the like in order toobtain optimum results; however, other suitable materials including manysemiconductvie materials such as raw cellophane, which are ordinarilynot thought of as conductors but which are still capable of acceptinginjected charge carriers of the proper polarity under the influence ofthe applied field may be used within the course of the presentinvention. The use of more conductive materials allows for cleanercharge separation and prevents possible charge buildup on the electrodewhich tends to diminish the interior electrode field. The blockingelectrode on the other hand is selected so as to prevent or greatlyretard the injection of electrons into the photosensitive pigmentparticles when the particles reach the surface of this electrode. Theblocking electrode base or core generally will consist of a materialwhich is fairly high in electrical conductivity. Typical conductivematerials utilized are conductive rubber, and metal foils of steel,aluminum, copper and brass. If, however, a low conductivity material isused a separate electrical connection may be made to the back of theblocking layer of the electrode. Although the blocking electrodematerial need not necessarily be used in the system the use of such alayer is preferred because of the markedly improved results which it iscapable of producing. Generally speaking, the blocking electrodematerial or layer consists of a material having a resistivity of aboutohms cm. or greater with the preferred materials having a resistivity inthe range of from about 10 to 10 ohms cm. EX- emplary of the preferredblocking layer materials used are baryta paper, which consists of papercoated with barium sulfate suspended in a gelatin solution, Tedlar, apolyvinylfiuoride, and polyurethane. Any other suitable material havingthe desired resistivity properties may be utilized. Typical materialsinclude cellulose acetate coated papers, polystyrene,polytetrafiuoroethylene, and polyethyleneterephthalate. The blockingelectrode layer when utilized may be a separate replaceable layer whichis easily fixed in some manner to the blocking electrode such as bymechanical fasteners or any other similar device which is capable ofsimply holding the layer on the electrode. In the alternative, the layermay be an integral part of the electrode itself being adhesively bonded,laminated, spray coated or otherwise applied to the surface of theelectrode.

Where the above imaging steps are repeated with cleaning of the blockingelectrode but without discharging the blocking electrode beforerepetition of the imaging step it has been found that there is a steadydecrease in the image quality of the successive copies. It has beenfound that this general gradual decrease in image quality is due to theaccumulation of undesired electrostatic charge on the surface of theblocking electrode. Therefore, in accordance with the present invention,the voltage supplied to the system is applied by way of the coatingmeans used which itself makes rolling contact with the blockingelectrode layer or material on the surface of the roller electrode core.Generally, a potential of about 3,000 to 7,000 volts has been applied tothe coating means so as to establish the necessary field across theimaging suspension during imaging. Preferred operating potentialsgenerally fall within the range of from about 4500 to about 5500 volts.The coating device utilized may comprise at least more than one rollerto disperse this suspension on the imaging or blocking electrode.However, the number of rollers utilized in conjunction with the imagingapparatus will be determined by the particular application of theimaging system with respect to the degree to which the suspension is tobe coated on the imaging electrode. Representative coating thicknessesrange from about 1 to about 6 mils with a coating thickness of about 4mils generally being preferred. Any suitable coating rod may be usedsuch that it will transmit upon contact with the blocking electrodesurface a potential which both neutralizes charge buildup on theblocking electrode layer and establishes the necessary electric fieldacross the imaging suspension at the area of contact between theblocking and injecting electrodes. Typical coating rods which may beused include a conductive glass coating rod, a conductive rubber coatingrod, and a wire wound metal coating rod otherwise known as a Meyer rod.

Although various electrode spacings may be employed,

spacings of less than 1 mil and extending down even to the point wherethe electrodes are pressed together constitutes a particular preferredform of the invention in that this configuration produces betterresolution and, in a polychrome imaging system, superior colorseparation results. This improvement is believed to take place becauseof the high field strength across the' suspension during imaging.

In the polychromatic system, the particles are selected so that those ofdifferent colors respond to different wavelengths of light in thevisible spectrum corresponding to their principal absorption band andfurther so that their spectral response curves do not have substantialoverlap thus allowing for color separation and subtractive multicolorimage formation. Several different particles are employed namely a cyancolored particle sensitive mainly to red light, a magenta coloredparticle sensitive mainly to green light and a yellow colored particlesensitive mainly to blue light. Although this is the simplestcombination, additional particles having different absorption maximummay be added to improve color synthesis. When mixed together in thecarrier liquid, these particles produce a generally black liquid andwhen one or more of the particles are caused to migrate from thesuspension the remaining particles produce a color equivalent to thecolor of the impinging light source. Thus, for example, red lightexposure causes the cyan colored pigment to migrate thereby leavingbehind the magenta and yellow pigment which combine to produce red inthe final image. In the same manner, blue and green color is produced byremoval of yellow and magenta pigment respectively and of course whenwhite light impinges upon the mix all pigments migrate leaving behindthe color of the white or the transparent substrate. No exposure leavesbehind all pigments which combine to produce a black image. It should berecognized that this is an ideal technique of subtractive color imagingin that the particles are not only each composed of but one componentbut in addition they perform a dual function in that they act both asthe final image colorant and as the photosensitive medium of the system.Accordingly, the system represents virtually the ultimate' ineliminating the complexity of prior art methods of subtractive colorimaging. In a monochromatic system, particles of a single color orparticles of more than one color but responding to substantially thesame wavelength of light are dispersed in the carrier liquid and exposedto a black and white image. A single colored image results correspondingto black and white photography.

The roller blocking electrode configuration shown in the drawing is ofcourse merely representative, and any other similar configuration may beused. For example, the blocking electrode may be in the form of amovable or stationary flat plate or in the form of a continuous belt. Itis also possible to provide a transparent blocking electrode with theimaging or exposure lamp being located inside the respective electrode.

Any suitable carrier liquid may be used in the course of the presentinvention. Materials found suitable include decane, dodecane andtetradecane, molten paraffin wax, molten beeswax and other moltenthermoplastic materials. Sohio Odorless Solvent, a kerosene fractioncommercially available from the Standard Oil Co. of Ohio and *Isopar G,a long chain saturated aliphatic hydrocarbon commercially available fromthe Humble Oil Co. of New Jersey have been found suitable. Mixtures ofthe above defined compositions have also been found to satisfy therequirements of the present invention.

Any suitable differently colored photosensitive pigment particle havingspectral responses such as disclosed in US. Pats. 3,384,488 and3,384,566 may be used to form the imaging suspension of the presentinvention. The percentage of pigment in the insulating liquid carrier isnot considered critical; however, for reference purposes it is notedthat from about 2 to about 10 percent pigment by weight has been founddesirable to produce acceptable results. As previously stated once theparticle image is formed it may be fixed to the respective electrodessuch as by spraying a binder onto the surface, by laminating an overlayover the imaged surface or by including a binder in the liquidsuspension medium. Generally, it will be found preferable to transferthe image from the electrode and fix it to a secondary surface so thatthe electrode may be reused. The transfer step may be carried out by anadhesive pick off technique, such as with adhesive tape, or preferablyby electrostatic field transfer. Transfer may also be affected byphotoelectrophoretic means.

To further define the specifics of the present invention the followingexamples are intended to illustrate but not limit the subject matter ofthe present invention. Parts and percentages are by weight unlessotherwise indicated.

PREFERRED EMBODIMENTS In the following examples four different imagingsuspensions are employed utilizing the apparatus of the general typeillustrated in FIG. 1. The injecting electrode is made up of NESA glassand the surface of the glass connected to ground. The blocking electrodeconsists of a hard conductive rubber core approximately 2 inches indiameter coated with a layer of Tedlar. A negative potential of about3,000 volts is imposed on the coating rod during exposure. Exposure ismade through a photographic transparency with a light intensity of about1800 foot candles. The imaging suspension used in this examples consistsof about 17 parts by weight of the pigment particles dispersed in about100 parts mineral oil. A wire wound metal rod is used as the coating rodin Examples 1 and 2 and a conductive rubber roller is used in Examples 3and 4. The imaging suspension is coated on the surface of the blockingelectrode to a thickness of about 3 mils for Examples 1 and 2 and about4 mils for Examples 3 and 4.

Example I A tri-mix is prepared by dispersing equal parts of a cyanpigment, Diane Blue, 3,3-methoxy-4,4'-diphenyl-bis-(1-azo-2-hydroxy-3"-naphthanilide), CJI. No. 21180, available fromHarmon Colors, a magenta pigment, Ca1- cium Lithol Red, a calcium lakeof 1-(2-azonaphthalene- 1-sulfonic acid)-2-napthol, C.I. No. 15630,available from Collway Colors; and a proprietary yellow pigment, N 2"pyridyl-8,-13-dioxodinaphtho-(2,l-6,2',3-d-furand-carboxyamide,discolsed in U.S. patent application No. 421,281, filed Dec. 28, 1964now Pat. No. 3,447,922 and having a common assignee, in mineral oil. Thetri-mix is imaged as described above forming a full colored imageconforming to the color transparency.

Example -III Seven parts by weight of the metal-free phthalocyaninepigment disclosed in Example I is blended with 100 parts mineral oil.The resulting uni-mix is coated on a NESA electrode and imaged asdescribed above. A single color An imaging suspension comprising equalamounts of Bonadur Red B, 1-(4'chloro-5-ethyl-2'-sulfonic acid)azobenzene-2-hydroxy-3-naphthoic acid, available fromAmerican'Cyanamide; Monolite Fast Blue G.S., and the proprietary yellowpigment disclosed in Example I in Sohio Solvent 3440 is prepared andimaged as described above. A full colored image conforming to theoriginal color transparency is obtained.

Although the present examples were specific in terms of conditions andmaterials used, any of the above listed typical materials may besubstituted when suitable in the above examples with similar resultsbeing obtained. In addition to the steps used to carry out the processof the present invention, other steps or modifications may be used, ifdesirable. Other materials may also be incorporated in the imagingsuspension and other facets of the invention which will enhance,synergize or otherwise desirably effect the properties therein desired.For example, various sensitizers may be utilized in conjunction with theimaging suspension.

Anyone skilled in the art will have other modifications occur to thembased on the teachings of the present invention. These modifications areintended to be encompassed within the scope of this invention.

What is claimed is:

1. A photoelectrophoretic imaging apparatus comprising in combination arotatably mounted first electrode, conductive means to coat continuouslyand uniformly an imaging suspension on the surface of said firstelectrode, a second electrode adapted to contact said suspensionopposite said first electrode and spaced up to about one mil therefrom,at least one of said electrodes being substantially transparent, and atleast one of said electrodes having a blocking surface facing saidimaging suspension, means for applying a potential to the conductivecoating means thereby developing an electric field across saidsuspension between said electrodes and means to expose said suspensionto an image with actinic electromagnetic radiation, whereby an image isproduced.

2. The apparatus as disclosed in claim 1 wherein said coating meanscomprises at least one wire-wound metal rod.

3. The apparatus as discolsed in claim 1 wherein said coating meanscomprises at least one conductive glass rod.

4. The apparatus as disclosed in claim 1 wherein said coating meanscomprises at least one conductive rubber roller.

5. The apparatus as disclosed in claim 1 wherein said second electrodeis rotatably mounted in close proximity to said first electrode, suchthat the surface of said second electrode is in area contact with thesuspension on the surface of said first electrode, the axis of saidsecond electrode being parallel to the axis of said first electrode, andfurther including means to rotate said first and second electrodessynchronously.

References Cited UNITED STATES PATENTS 2/ 1969 Mihajlov 2O4-300 3/1971Fisher et al. 355--3 U.S. Cl. X.R. 3553

